This invention relates to an R.F. oscillator arrangement comprising a diode operable to produce pulses of radio frequency energy when pulses of direct current greater than a critical current are applied to the diode, the radio frequency being dependent on the temperature of the diode.
The diode is suitably a TRAPATT diode.
Present theory and experience indicate that the TRAPATT mode is a large-signal mode that develops from a small-signal negative resistance. There is a critical bias current below which negligible, if any, microwave energy is generated. This critical current is high. A typical S-band p-n junction TRAPATT diode with a junction area of 4.times.10.sup.-4 cm.sup.2 has a critical bias current of about 1-2 amps when mounted in an oscillator circuit that is typically of the time-delay-triggered (T.D.T.) type, such as that described by W. J. Evans in "Circuits for high-efficiency avalanche-diode oscillators", I.E.E.E. Trans. MTT-17, 1060, (1969), or as described in U.K. patent applications Nos. 2026800 and 2032715 corresponding to U.S. Pat. Nos. 4,348,646 and 4,354,165, respectively.
In common with other oscillators the frequency of free running TRAPATT oscillators, varies with the diode and circuit temperatures which are of course functions of the ambient temperature. Both coaxial and microstrip S-band (about 2.5 GHz) TRAPATT oscillators have been tested by the applicant over wide temperature ranges, for example -50.degree. C. to +100.degree. C., and the frequency-temperature characteristic has been negative and typically approximately linear with a value of between about -300 kHz/.degree.C. and -400 kHz/.degree.C. for flip-chip diodes.
The temperature of the circuit affects the operating frequency because of the coefficient of linear expansion of the metal structure of the oscillator and the temperature dependence of the permitivity of any dielectric material in the delay-line associated with the T.D.T. circuit. Increasing temperature increases the length of the oscillator structure and generally increases the permitivity of dielectrics in the delay line such as polystyrene supports in a coaxial oscillator or the alumina substrate in a microstrip version; both of these effects cause the oscillation frequency to decrease with increasing temperature. However, the variation of frequency calculated from the temperature dependence of these parameters is considerably less than that observed in such oscillators (which is about -300 to -400 kHz/.degree.C.).
The temperature of the diode device also determines the oscillation frequency. It is suggested that the effect of temperature on the device is such that, assuming a charging current that is independent of temperature, the time taken each R.F. cycle for the particle current to increase by avalanche multiplication to such a level that the displacement current is zero varies with diode temperature. The effect of this temperature dependence can be best appreciated by considering the device voltage-time characteristic. While the diode voltage is below the breakdown voltage the diode appears as a capacitor. Assuming the diode to be instantaneously biased below breakdown, a constant charging current will cause the voltage across the diode to increase linearly with time. As the voltage passes through breakdown and continues to increase with time, the number of charge-carriers in the depletion region starts to increase by avalanche multiplication. The voltage across the diode continues to increase until there are sufficient carriers to carry all the charging-current (at this point the displacement current is zero); the diode voltage has now reached a maximum from which it falls rapidly with the further increase in the number of carriers due to avalanche multiplication. (In a T.D.T. circuit, this collapse launches the trigger pulse into the delay-line circuit along which the pulse travels, the pulse polarity being inverted at a step transition and returning along the delay line to provide the next trigger. We have seen earlier that the circuit delay varies only slightly with temperature). The time between the diode voltage passing through breakdown and reaching a maximum varies with temperature considerably because of the ionization coefficients of holes and electrons, the saturated drift velocity and the saturation current; each of these parameters is a function of temperature, ionization and saturated drift velocity decreasing with increasing temperature while saturation current increases with increasing temperature. It is thought that the net negative frequency-temperature characteristic of TRAPATT diode oscillators is due to negative contributions from the ionization and saturated drift velocity and a positive contribution from the saturation current.
It has been proposed (see the published abstract of Japanese patent application No. 55-23628) to reduce the range of frequency variation of the pulse of a pulsed microwave oscillator by providing a temperature sensing element on the heat-sink of the oscillation element and using it to control the supply of electrical power to adjacent heating elements, the heat-sink and heating elements being mounted on a part made of thermal insulator; the ON and OFF set value of the temperature sensing element is selected slightly higher than the estimated ambient temperature. However, since the efficiency with which a direct current is converted to R.F. energy in a microwave oscillator is generally very much less than 100%, the production of a substantial amount of R.F. energy also involves the production of a substantial amount of heat which must be removed from the oscillation element to prevent it from becoming too hot. The above-mentioned arrangement has the major disadvantage that the thermal insulator inhibits the loss of heat from the oscillation element; very little heat can be produced in the oscillation element without its temperature rising considerably above the temperature at which the temperature sensing element is set to operate, thereby degrading performance, particularly the operating lifetime of the oscillation element. Moreover, if material of good thermal conductivity were to be used instead of the thermal insulator, a large amount of power would be required to maintain the heat-sink at the desired temperature when the ambient temperature was low. It may also be noted that a simple ON-OFF control of a heating current of fixed magnitude is not well suited to accurate maintenance of a preset temperature in conditions where the ambient temperature may vary over a wide range.